Machines that remove the upper layer of pavement from a concrete road employ a plurality of cutting tools mounted on a drum, with each of the tools rotatable about its longitudinal axis. When such machines are employed to remove the upper surface of a road, the tools become worn and must be periodically replaced. Depending upon weather conditions, it may be necessary to replace the tools mounted on the drum of such machines daily, and sometimes twice daily. The drums of such machines typically mount more than one hundred of such tools, and therefore, the machines that remove the upper surface from a road must be removed from service for a lengthy period of time while the tools on the drum are replaced. The time loss that occurs while tools are replaced contributes significantly to the cost of resurfacing roads.
Similarly configured tools are used in trenching machines and rock saws for cutting grooves in concrete and the replacement of the tools on these machines increases the costs of operating these machines.
It is desirable, therefore, to extend as long as possible the useful life of the tools mounted on such machines. In order to extend the useful life of the tools, the manufacturers of such tools are engaged in a heated competition to find a configuration of a tool body with improved endurance to wear.
The tools mounted on such machines have an elongate metal body symmetric about a longitudinal axis and consist of a cutting end at the forward end of which is a seat for receiving a hardened tip, and behind the cutting end is an elongate cylindrical shank which is rotationally received in a cylindrical bore of a tool holder. The failure of such tools can be classified into certain clearly defined categories. First, tools may fail as a result of fracture of the hardened tip. Second, the braze that retains the hardened tip in the seat at the forward end of the tool may fail, such that the tip becomes dislodged from the tool. The tool may also fail because of washaway of the metal from which the tool body is made. Finally, a tool may fail because the hardened tip at the forward end of the tool has become dull and the tool can no longer effectively cut the hard surface against which the tools on the drum are directed. The manufacturers of such tools have been seeking a configuration of a cutting tip and tool body that will maximize the useful life of the tool.
One way of reducing the washaway of the steel bodies of such tools is to provide an enlarged tungsten carbide tip at the forward end of the tool. The most expensive portion of such tools, however, is the tungsten carbide from which the hardened tip is constructed and therefore providing a tool with an enlarged cutting tip greatly increases the cost of the tool. Furthermore, it has been found that a tool having an enlarged diameter tip will not maintain a sharp configuration for an extended period of time and therefore, although the tool does not suffer from washaway, it must be prematurely discarded when the cutting tip has become dull.
One configuration of a cutting tip which has had recognized success is disclosed by Ojanen, U.S. Pat. No. 4,497,520. The Ojanen tip has a tapered forward end, a generally frustoconical midsection that diverges gradually along its length, followed by an enlarged diameter base with a fillet between the frustoconical midsection and the base.
The enlarged diameter base of the Ojanen tip provided an enlarged surface area for bonding the tip into the seat at the forward end of the tool. During use, the material that forms the sharpened forward end of the cutting tip is gradually worn away. The elongate mid-portion of the tip gradually becomes shortened, but the outer diameter surrounding the sharpened portion of the tip remained substantially the same because of the gradual incline of the frustoconical mid-portion. As a result, even though material has eroded away from the surface of the tip, the tip generally remained relatively sharp and the tool continued to be useful as the carbide of the mid-portion is not worn away. It is not until the material that comprises the fillet between the midsection and the base begins to wear away that the tip will become dull and no longer useful.
In the meantime, however, existing tools employing tips embodying the configuration of the Ojanen tip have generally suffered greatly from washaway. Generally, long before a cutting tip embodying the configuration disclosed by Ojanen has become dull, washaway has so eroded the central body of the tool that the tool has acquired an hourglass configuration, and breakage of the tool body could occur between the base of the cutting tip and the shank.
On milling machines used to remove the surface of pavement, the tools are mounted on a rotating drum with the tools positioned on the drum to cut grooves in the surface of the asphalt or concrete with each of the grooves cut by the cutting tip of one of the tools. The tools of a milling machine are mounted on the drum in a spiral configuration and positioned to form grooves having a distance of approximately five-eighths inch between the center lines of adjacent valleys of the grooves with a solid ridge of material between the adjacent valleys of each of the grooves.
I have observed that it is the ridges between the valleys of the asphalt or concrete surface that are responsible for causing a great deal of the washaway that leads to the hourglass configuration of the tool body. It has thus been apparent to me that a tool body would be less subject to washaway if the tools could be configured so as to cut grooves in the hardened asphalt or concrete with less pronounced ridges. On the other hand, it would be desirable to provide a tip which would offer certain protection to the tool body behind the tip without greatly increasing the mass of tungsten carbide material from which the tip is made, such that the cost of the tip is not substantially increased.